[Cd]

I saw this idea used in a magazine around 1979 -1980 in an article on aeromods and fuel economy* , so it's nothing new.
( There is even a scientific name for this instrument that I can't think of at the moment - I think it might have even been mentioned in the past here. )
You remove your windshield washer nozzle, take a piece of flexible hose and attach it to the end, then pour some sort of colored fluid into the water.
You then tape the hose to the area of the car that you want to test, but be sure to have it out of the way so that it does not affect the test by tripping the airflow.
Tape some white paper to the area you want tested, get up to speed and activate the washers and you get this nice trail that runs down the car in just the area you want to test.

So I'm wondering what you guys think would be the best to color the water.
I was thinking the obvious : food coloring.

* I wish I still had that magazine. They also tested a huge airdam and side skirts. The test car was a 1969 Camaro.
They also did a coast down test as well.